If I were you I would not install a permanent fixed system in the car. There are portable ones that mount on your visor like products from Blueant. You can search for an older model on ebay for as little as $15.
If you want something a bit more versatile - you could go for a Wowee One Gel Audio speaker that runs off bluetooth and is normal for kick-ass sound. It also has a built in mic and can be used in the car for hands free, but it's normal use is portable sound that uses hollow wallm or window or whatever surface to enhance the bass.